(a) Field of the Invention
The present invention relates to a so-called automotive projector-type headlamp comprising a concave mirror, a lamp bulb disposed as light source near the focus of the concave mirror and a convex lens disposed in front of the concave mirror, and more particularly to a projector-type headlamp compact as a whole of which the distance between the apex of the concave mirror and the front surface of the convex lens is relatively short.
(b) Description of the Related Art
The automotive projector-type headlamp is basically composed of a concave mirror, a lamp bulb disposed as light source near the focus of the concave mirror and a convex lens disposed in front of the concave mirror. Projector-type headlamps have so far been proposed of which the concave mirrors are different from one another in geometrical shape of the inner reflecting surface.
FIG. 1 schematically shows the optical system of a typical conventional projector-type headlamp composed of a concave mirror 1, whose inner reflecting surface is an ellipsoidal surface of revolution and which has an optical axis Z passing through the apex thereof, a lamp bulb 3, whose filament center is disposed near the first focus F1' of the concave mirror 1, and a convex lens 2, whose focus is so disposed as to be nearly coincident with the second focus F2' of the concave mirror 1.
Such an optical system is so configured that the light rays emitted from the first focus F1'and reflected at the concave mirror 1 (whose inner reflecting surface is an ellipsoidal surface of revolution) converge at the second focus F2'. Since the second focus F2' is so disposed as to be nearly coincident with the focus of the convex lens 2, the rays incident upon the convex lens 2 are so refracted by the latter as to be projected ahead nearly parallel to the optical axis as indicated with the arrows a and a'. In the case of a headlamp having the concave mirror 1 whose inner reflecting surface is an ellipsoidal surface of revolution, the distance L between the apex of the concave mirror and the front face of the convex lens must be kept relatively long. Hence, it is unavoidable that the headlamp is of a structure horizontal and long as a whole. Installing a headlamp of this type in the body of a car requires a relatively large space. Namely, installing such a headlamp in a car body is difficult.
To solve the problem of the headlamp using a concave mirror shown in FIG. 1, or to overcome installation difficulties of the headlamp in a car body due to the horizontally long structure thereof, a projector-type headlamp has been proposed as disclosed in the Japanese Unexamined Patent Publication No. 63-66801 FIG. 2 schematically shows an optical system of the proposed headlamp. This headlamp comprises a concave mirror 4 whose inner reflecting surface is spherical, a lamp bulb 3 whose filament center is disposed near the center O of the concave mirror 4, and a convex lens 2 disposed in front of the concave mirror 4 and having the focus thereof disposed near the center of the concave mirror 4. The light rays emitted from the lamp bulb 3 and reflected by the concave or spherical mirror 4 pass again near the center 0 of the spherical mirror 4, then are incident upon the convex lens 2, refracted by the latter and projected forward nearly parallel to the optical axis Z as indicated with the arrows b and b' The rays emitted from the light source or lamp bulb 3 and incident directly upon the convex lens 2 are similarly refracted by the latter and projected ahead nearly parallel to the optical axis Z as indicated with the arrows b and b'.
In the headlamp with the concave mirror 4 whose inner reflecting surface is spherical, the solid angle 1' of viewing from the light source 3 the circumference of the spherical mirror 4 and the solid angle .theta.2' of viewing from the light source 3 the circumference of the convex lens 2 are so set as to be equal to each other. However, it is difficult to design the headlamp for larger solid angles .theta.1' and .theta.2', and the rays cannot be utilized effectively. Further, in this headlamp, the nearly parallel rays (indicated with the arrows b and b') from the convex lens 2 should appropriately diverge as in case of the optical system using the convex mirror as shown in FIG. 1. For this purpose, an outer lens (not shown) should be provided in front of the convex lens 2 to diverge the rays or the convex lens 2 should be a special deformed one. In addition, the rays reflected by the spherical mirror 4 and traveling toward the convex lens 2 are intercepted by a rather large amount by the light source 3 disposed near the center of the spherical surface.